Transparent electrode for electrophoretic imaging

ABSTRACT

Apparatus for improved imaging and for eliminating corona arcing in an electrophoretic imaging system employing a transparent electrode having electrically conductive transparent portions causing variations in the surface potential of the electrode under the influence of an electrical field. The electrode functions within an imaging system also having a means to illuminate the conductive portions for forming an image in a confined electrical field in a manner preventing corona arcing between electrodes of the imaging system.

United States Patent i 1 Gundlach [111 3,744,897 [4 1 July 10,1973

[ TRANSPARENT ELECTRODE FOR ELECTROPHORETIC IMAGING [75] Inventor: Robert W. Gundlach, Victor,N.Y.

[73] Xerox Corporation, Rochester,

Filed: May 26, 1971 Appl. No.: 147,179

Related US. Application Data Division of Ser. No. 821,369, May 2, 1969, Pat. No. 3,630,884.

Assignee:

US. Cl 355/3, 96/1 R, 204/181, 204/299, 204/300 Int. Cl 603g 15/22 Field of Search 204/181 PE, 299 PE, 204/300 PE, 18 PC, DIG. 7; 355/3; 95/1 A; 96/ 1.5, l R

[56] References Cited UNITED STATES PATENTS 3,653,890 4/1972 Seimiya et al. 355/3 Primary Examiner-John H. Mack Assistant ExaminerW. 1. Solomon Attorney-James J. Ralabate electrical field in a manner preventing corona arcing between electrodes of the imaging system.

9 Claims, 4 Drawing Figures PAIENIED JUL '1 0 1m sumaarz TRANSPARENT ELECTRODE FOR ELECTROPHORETIC IMAGING This is a division of application Ser. No. 821,369, filed in the United States, May 2, 1969, now US. Pat. No. 3,630,884.

This invention relates in general to imaging systems and more specifically to an improved electrophoretic imaging system.

The system improved by this invention is of the type using photosensitive radiant energy absorbing particles believed to bear a charge when suspended in a nonconductive liquid carrier and disposed in an electroded system to be exposed to an image radiation configuration. For a detailed description of the operation of this system see US. Pat. Nos. 3,384,565 issued to V. Tulagin and L. M. Carreira, 3,384,566 to H. E. Clark and 3,383,993 to S. Yeh issued on May 21, 1968. The particles of the system migrate in image configuration providing a visual image at one or both of the electrodes between which they are placed. The system employs particles which are photosensitive and which apparently undergo a net charge alteration upon exposure to activating radiation by interaction with one of the electrodes. Various mixtures of two or more different colored particles can be used to secure various colors of images and imaging mixes having different spectral response. These colors can be used independently or in subtractive color synthesis. In a monochromatic system the particles will migrate if energy of any wavelength within the panchromatic spectrum of the particle response strikes the particle.

It has been found that images produced by the system broadly described above may on occasion exhibit uneven density or contrast. Further, the apparatus employed for imaging may be damaged during imaging. It is thought that these difficulties are caused by varying corona discharge or air ionization, referred to hereinafter as corona arcing, between the electrodes used for imaging within the system as one electrode approaches in proximity to the other electrode. The invention herein was developed to eliminate this electric arcing between electrodes while improving sensitivity levels of the imaging systems in which it is used. The invention enables a system in which it is used the capability for increasing field strength during imaging at the most effective position for imaging without the consequence of increased corona discharge between the electrodes of the system.

Therefore, an object of this invention is to improve electrophoretic imaging systems by eliminating corona arcing between electrodes. Another object of this invention is to improve systems using transparent electrodes. Still another object is to improve transparent electrodes capable of eliminating corona arcing in imaging systems.

The foregoing objects and others are accomplished in accordance with this invention by providing a transparent electrode for contact with another electrode within an electrophoretic imaging system wherein the transparent electrode has a variation of electrical conductivity across its surface. Corona arcing is prevented because the field is at least reduced in areas where arcing is likely to occur. Also corona arcing is prevented because very high local fields can be reached (around the line or point conductors) without requiring the usual high voltage across the entire gap. In one embodiment a commutating means contacts a portion of the conductive surface to provide a potential thereto. It may be that other systems exist or will be discovered or invented that require improvements similar to those described herein and this invention can be used thereon to improve such a system and such use is contemplated hereby.

The advantages of this improved transparent electrode and electrophoretic imaging system will become further apparent upon consideration of the following detailed disclosure of the invention; especially when taken in conjunction with accompanying drawings; wherein:

FIG. 1 is a side view schematic of an electrophoretic imaging system employing one embodiment of this invention;

FIG. 2 is a sectional side view schematic illustration of another embodiment;

FIG. 3 is a perspective view of a transparent electrode;

FIG. 4 is a sectional side view of a portion of a modified embodiment of this invention.

Referring to the drawings, shown in FIG. 1 is a photoelectrophoretic imaging system having an injecting electrode 10 made in accordance with this invention. It is composed of a transparent ground or etched glass substrate 12 and an overcoated, transparent electrical conducting layer 14. The overcoated conductive layer 14 may be a layer of tin oxide coating or any other coating which is both transparent and electrically conducting deposited on glass substrate 13 to provide an electrode or plate.

Deposited on the electrical conducting layer 14 of the injecting electrode 10 is a thin layer of finely divided photosensitive particles dispersed in an insulating liquid carrier. This is the imaging suspension 16 from which an image is formed.

Adjacent to the electrode 10 and the suspension 16 coated thereon is an imaging electrode generally referred why the numeral 18. The imaging electrode has a layer 20 of blocking material, that is, material which once contacted by photosensitive particles will not inject a sufiicient charge into them to cause them to migrate from the imaging electrode. The inner roller 21 is electrically conductive.

The term photosensitive for the purposes of this invention refers to the properties of a particle which, once attracted toward the injecting electrode, will reverse its polarity of charge and migrate away from it under the influence of an applied electric field when exposed to activating electromagnetic radiation. The term suspension may be defined as a system having solid particles dispersed in a solid, liquid or gas. Nevertheless, the suspension described in the embodiments herein is of the general type including those having a solid suspended in a liquid carrier. The term injecting electrode" refers to the electrode the properties of which apparently inject charges into photosensitive particles activated by electromagnetic radiation while under the influence of an electric field.

The photosensitive suspension 16 is subjected to electromagnetic radiation in image configuration by, for example, shining a light source 22 through an object such as a transparency 25 which is imaged through lens 26 to the photosensitive suspension 16 on the upper surface of the injecting electrode 10. More or less simultaneously with the projection of the object 25 to the suspension 16, an electric field is applied between the electrodes and 18. The imaging electrode is given shape by the conductive support member 21 and the shaft 23 which is suitably journaled for rotation over the injecting electrode 10. Shown schematically in FIG. 1 is an electrical energy source 28 connected through a switch 30 to the conductive roller 21 of the imaging electrode 18. The mechanism functions such that when the imaging electrode 18 traverses the surface of the injecting electrode 10, the switch 30 is closed forming an electric field at the interface of the two electrodes. The surface of imaging electrode 18 is preferably negative relative to the surface 14 of the injecting electrode 10.

The particles within the suspension are nonconductive unless they are struck with activating radiation. Under the influence of .the applied electric field, as shown in this embodiment, the negative particles come into contact with or are closely adjacent to the injecting electrode 10 and remain there. When activating radiation strikes the photosensitive particles, it makes the particles conductive creating holeelectron pairs of charge carriers which may be considered mobile in nature. These newly created holeelectron pairs within the particles are thought to remain separated before they can re-combine due to the electrical field surrounding the particles between the two electrodes. .The negative charge carriers of these hole-electron pairs move toward the positive electrode 10 while the positive charge carriers move toward the negative imaging electrode 18. The negative charge carriers near the particle-electrode interface at electrode 10 can move across the very short distance be tween the particles and the surface 14 leaving the particles with a net positive charge after sufficient charge transfer. These net positively charged particles are now repelled away from the positive surface of the electrode 10 and are attracted toward the negative imaging electrode 18. The particles struck by activating radiation of a wavelength to which they are sensitive, i.e., one which will cause the formation of hole-electron pairs within the particles, move away from the electrode 10 to the electrode 18 leaving behind only particles which are not exposed to sufficient electromagnetic radiation in their responsive range to undergo this change. Those particles remaining function to form the image sought by the imaging system.

The conductive surface 14 of the injecting electrode 10 is shaped, in one embodiment, to have various peaks and troughs as shown in the cross section of FIG. 1. The 7 peaks and troughs might actually be formed in the substrate 12 and coated evenly by the conducting surface 14. On the other hand, it would also be possible to form the peaks and troughs with the transparent conductive layer [4 alone overlayed on a flat surface. By shaping the surface of the injecting electrode as done by this invention, the field strength of the imaging system at its most important area, that which is between the electrodes, may be increased without deteriorating effects on the image due to corona arcing between the electrodes. The transparent conductive surface may be formed to have points, lines or ridges either physically in its surface or in its conductivity pattern.

What occurs during the imaging of the photoelectrophoretic suspension 16 is that the lines of force, and therefore the path of charged particles, will converge to the peaks of the conductive injecting electrode surface 14. This is schematically shown by the lines of force 24 shown between the two electrodes of FIG. 1. Therefore, at the suspension-injecting electrode interface the particles migrate toward the protrusions of the injecting electrode surface where the lines of force of the field are the greatest and they exchange charge thereat before reversing their direction to migrate to the imaging electrode surface 20. By peaking the field force at the protrusions of the injecting electrode 10 and having the field across the surface 20 of the imaging electrode 18, the probability of having corona arcing between the two is lessened. The reason for this is that one may use a lesser field to achieve the same imaging results due to the shaping of the lines of force, or alternatively one may use the same electrical field as with prior art photoelectrophoretic imaging systems and due to the shaping of the force field the force will be gathered more inthe areas where the suspension is located then in those areas encompassing the air gap between the two electrodes.

FIG. 2 shows an alternative embodiment of the invention herein in an imaging system adapted for continuous imaging of photoelectrophoretic pigments. Here, the injecting electrode 10 is formed as a cylinder with a substrate 32 having conductive portions 34 on its outer surface. The outer surface of the injecting electrode 10 may be smooth, having areas of varying conductivity along the surface without physical peaks and troughs therein. The peaks and troughs are of electrical conductivity. This is beneficial to facilitate making uniform ink layers for imaging and to prevent the suspension from settling into the troughs between the ridges of conductive material. This naturally will make image transfer and cleaning of the injecting electrode surface easier physical operations. To this end, an insulating resin could be doctored into the depression between the peaks of conductive material and left there. Instead of forming the conductive layer of the insulating transparent surface by attaching or bonding conductive or semi-conductive wire or strips to the surface, a conductive pattern of chromium, copper, aluminum, or any other suitable conductor or semi-conductor may be evaporated through a screen or grid. For apparatus such as schematically shown in FIG. 1, it would be desirable to have a screen or grid pattern, either physical or electrical, that interconnects the various points or ridges so that the entire injecting electrode can be grounded or suitably electrically connected to permit the proper field for imaging across its entire surface during the contact with the imaging electrode 18.

The imaging electrode 18 has a conductive inner core 36 and a blocking outer surface 38. Imaging suspension 16 is deposited on one of the electrodes, here shown to be the imaging electrode 18, by dispensing through suitable means from a reservoir such as held in container 40.

In order to form the images, a stationary mirror 41 is located within the rotary injecting electrode 10 to receive the image projected from a transparent object 42 and to direct the image through an exposure slit 43 onto the surface of the suspension at the area of contact between the two electrodes. The transparency 42 is shown as a film roll or web and is moved synchronously with the rotating injecting and blocking electrode cylinders to provide a flowing image. The image is projected through a lens 44 for imaging at the intersection of the two electrodes at the slit 45. The image is preferably projected in a plane normal to the surface of the drums so that distortion of the image at the image plane is held to minimum. The injecting electrode is grounded by some suitable connection between the conducting surface ridges or points and ground. The blocking electrode is connected to a potential source 46 such that a field exists across the suspension between the two electrodes with the blocking electrode being shown as positive. It is optically beneficial to have materials for the conductive strips 34 and the substrate 32 of optically similar properties.

As the two electrodes rotate in the direction shown by the arrows, a flowing image is formed in the nip between them. A portion of the suspenion 16 will form a transferable suspension image 47 which remains on the injecting electrode 10 while the non-image portion 48 of the suspension is carried away on the imaging electrode surface. The developed image is carried on the injecting electrode 10 for contact with a transfer material 49 which may be an adhesive web or other suitable transfer material. The web 49 moves along a predetermined path through pinch rollers 50 and 51 and around a transfer roller 52. At the transfer roller 52, the transfer material 49 is pressed into contact with the injecting electrode 10 in order to pick off the image portion 47 of the imaging suspension. The web is then passed through a fixing unit 53 which securely fixes the image 47 to the web 49.

As the electrodes continue to rotate the portion of the suspension 48 which does not form part of the image but remains on the imaging electrode 18 is carried to a cleaning station where suitable means such as schematically represented by a brush 54 removes the suspension from the imaging electrode surface. The cycle may now continue to rotate with other images being formed in the same manner as that described hereinabove.

FIG. 3 shows a perspective view of an injecting electrode suitable for use in the apparatus of FIG. 2. However, the transparent conducting strips of this injecting electrode are not interconnected with one another and therefore must be electrically contacted individually in order to enable field to exist between it and the imaging electrode at the interface of the two of them. The transparent substrate 56 of this injecting electrode is etched or formed in some manner to have fine closely spaced grooves therein to accept transparent conductive layers 57 therein. The outer surface of the insulating substrate 56 and the conductive layers 57 is polished to be completely smooth. The injecting electrode 10 has altemating conductive and insulating strips across its length. In this figure, as in all the others discussed, the dimensions are not to scale. The transparent injecting electrode surface will generally include many points or grooves per unit area depending for spacing on the particular method of deposition or screen pattern employed to form the conductive surface on a transparent substrate.

This electrode could be used in the embodiment of FIG. 2 by mounting it over a shaft 58 which would stably and immovably hold the reflecting mirror 41. A suitable frame would mount the injecting electrode 10 for rotation about the shaft without affecting the movement of the mirror or the connecting arm 59 which is grounded and connects a portion of the conducting injecting electrode surface to ground as it rotates past the connecting arm 59. The arm 59 thus acts as an electrical brush to make contact with a preselected portion of the conductive layers 57. The arm 59 is located at the same position as the slit 45 at the interface of the two electrodes. Therefore, in this embodiment the field exists in a limited area between the blocking electrode surface and a limited portion of the injecting electrode conductive layers 57 connected to ground by the arm 59.

By so limiting the effect of the field between the two electrodes this structure enables more efficient field utilization as described in the previous figures and further reduces the problem of corona discharge across the air gap between the two electrodes by eliminating the field existing across the air gap of the two electrodes. This is so because the connecting arm 59 is limited in contact to the portion of conductive strips 57 that is at or near the interface of the two electrodes and covered by imaging suspension during the imaging process.

FIG. 4 schematically represents a modified embodiment of an injecting electrode 10 for use in the photoelectrophoretic imaging system. In conjunction with the conductivity pattern disclosed for the injecting electrodes described above, optical sensitivity of the process is increased by converging the light from the projected image to the same minute areas on the sur face of the injecting electrode at which the electrical field is the greatest. Those areas, of course, being the peaks or ridges of conductive material on the surface of the injecting electrode 10. An example of increased optical efficiency in conjunction with the increased electrical field response efficiency is shown in FIG. 4 where a lenticulated surface 60 is formed into the injecting electrode on the side opposite the conductive surface used to contact the suspension. In this embodiment the light rays 62 eminating from the projected object strike the lenticular surface 60 of the injecting electrode and are converged to the conducting portion 64 of the injecting electrode. The lenticular surface can be a cylindrical lens system or any other light gathering means that can selectively gather light to'the particular locations of the conductive depositions. In this embodiment, the conducting portions 64 are placedon theinjecting electrode substrate 66 and an insulating solid material 68 is placed in the trough between the conductive portions of the injecting electrode. This provides a smooth surface for contact with the imaging suspension.

Various values can be used within the system to form images according to the known photoelectrophoretic process and reference is directed to the patents cited above for the various parameters of the process and the materials used therein. Various materials for forming conductive, semi-conductive and insulating areas within the injecting electrode can be used provided they function according to the requirements herein specified.

While this invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth since they are merely illustrative of the invention; and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or scope of the following claims.

What is claimed is:

l. A plate electrode which will produce a varying electrical field within an electrophoretic imaging contact zone established in an area between the plate electrode and a second electrode when a uniform electrical potential is applied across the two electrodes, the plate electrode comprising:

a. a transparent electrically insulating base; and

b. transparent electrically conductive means intimately attached to one surface of said base for varying the electrical field within an imaging contact zone when a uniform electrical potential is applied to said transparent electrically conductive means.

2. Plate electrode according to claim 1 wherein said conductive means comprises peaks and troughs of transparent electrically conductive material intimately bonded to said insulating base.

3. Plate electrode according to claim 2 wherein said troughs include relatively electrically insulating material overcoating the electrically conductive material therein to form a smooth surface.

4. Plate electrode according to claim 1 wherein said insulating base includes a plurality of grooves formed on one surface thereon.

5. Plate electrode according to claim 4 wherein said conductive means comprises a plurality of disconnected electrically conductive portions placed in the grooves of said insulating base.

6. Plate electrode according to claim 5 further comprising:

a. means to electrically isolate a portion of said plate electrode in the contact zone; and

b. means to electrically activate said isolated portion of said plate electrode to limit the applied electrical field between said isolated portion and a second electrode.

7. Plate electrode according to claim 1 wherein said conductive means comprises a plurality of disconnected, separated conductive portions intimately bonded to said insulating base.

8. Plate electrode according to claim 7 having relatively electrically insulating material placed between said separated conductive portions to form a smooth surface of alternating conductive and insulating portions thereon.

9. Plate electrode according to claim 8 further comprising a lenticular film on said transparent base on the surface opposite the surface having the transparent conductive means, said film positioned to the capability of gathering light striking it and refracting such light toward the conductive portions along the plate electrode surface. 

1. A plate electrode which will produce a varying electrical field within an electrophoretic imaging contact zone established in an area between the plate electrode and a second electrode when a uniform electrical potential is applied across the two electrodes, the plate electrode comprising: a. a transparent electrically insulating base; and b. transparent electrically conductive means intimately attached to one surface of said base for varying the electrical field within an imaging contact zone when a uniform electrical potential is applied to said transparent electrically conductive means.
 2. Plate electrode according to claim 1 wherein said conductive means comprises peaks and troughs of transparent electrically conductive material intimately bonded to said insulating base.
 3. Plate electrode according to claim 2 wherein said troughs include relatively electrically insulating material overcoating the electrically conductive material therein to form a smooth surface.
 4. Plate electrode according to claim 1 wherein said insulating base includes a plurality of grooves formed on one surface thereon.
 5. Plate electrode according to claim 4 wherein said conductive means comprises a plurality of disconnected electrically conductive portions placed in the grooves of said insulating base.
 6. Plate electrode according to claim 5 further comprising: a. means to electrically isolate a portion of said plate electrode in the contact zone; and b. means to electrically activate said isolated portion of said plate electrode to limit the applied electrical field between said isolated portion and a second electrode.
 7. Plate electrode according to claim 1 wherein said conductive means comprises a plurality of disconnected, separated conductive portions intimately bonded to said insulating base.
 8. Plate electrode according to claim 7 having relatively electrically insulating material placed between said separated conductive portions to form a smooth surface of alternating conductive and insulating portions thereon.
 9. Plate electrode according to claim 8 further comprising a lenticular film on said transparent base on the surface opposite the surface having the transparent conductive means, said film positioned to the capability of gathering light striking it and refracting such light toward the conductive portions along the plate electrode surface. 